This invention relates to a novel hydrophilic resin which has an oxazolidone-ring containing backbone and an ionizable group at the terminal end of the backbone. It also relates to a water-borne paint composition containing said resin.
It is known from Iwakura et al., J. Polymer Sci. Part A-1, 4 , 751 (1966) that an oxazolidone ring-containing modified epoxy resin may be produced by reacting a diglycidyl compound and a bisurethane, namely a monoalcohol-capped organic diisocyanate. The same resin may be produced by directing reacting a diisocyanate and the diglycidyl compound. Sander et al., J. Appl. Polymer Sci., 9 1984(1966). Since the resin is terminated with an epoxy group at each end and thus does not possess any ionizable or hydrophilic group, it is not usable in formulating electrodeposition paints and other water-borne paints.
European Patent Application Publication No. 0449072 discloses a method for producing epoxy resin-amine adducts used for the formulation of cathodic electrodeposition paints. According to this method, an oxazolidone ring containing compound produced by the reaction between a mono- or diglycidyl compound and an organic mono- or diisocyanate is directly subjected to a ring-opening reaction with an amine in conjunction with a conventional diepoxy resin. In use the electrodeposition pant containing this binder resin requires a relatively high voltage as high as 300 volt. When applied to the electrodeposition coating of galvanized steel sheets, the paint often fails to give a film of practical film thickness without occurrence of a number of pinholes due to the application of such a high voltage. This, of course, tends to degrade not only the appearance but also protective performance of the resulting finish. This European Patent also discloses a method Of producing the Oxazolidone ring-containing compound by directly reacting an organic diisocyanate and a glycidyl compound in the presence of lithium bromide catalyst. When directly reacting with the glycidyl compound, the diisocyanate tends to undergo a trimeization side reaction. If this side reaction occurred, the resulting modified resin would become too viscous and of less oxazolidone content. The alkali metal halide catalyst still remains in the modified resin. This is undesirable particularly when used in the electrodeposition coating because the presence of any electrolyte in the coating bath would adversely affect not only its electrodeposition performance such as coulomb efficiency and throwing power but also the corrosion resistance of films made therefrom.
The present invention is intended to eliminate or ameliorate these and other defects of the prior art resins and compositions.